3-methylpiperidine and 3-methylpyridine (3-picoline) are intermediates in the industrial production of nicotinic amide and nicotinic acid, which is an essential vitamin of the B-complex (vitamin B3). In this process, 3-methylpiperidine is converted to 3-methylpyridine in the presence of a dehydrogenation catalyst. The 3-methylpyridine is converted to 3-cyanopyridine by oxidative ammonolysis. The 3-methylpiperidine can be obtained by cyclization of 2-methyl-1,5-diaminopentane.
Catalysts function to increase the rate of a chemical reaction at a given temperature by lowering the necessary amount of energy to reach the transition state. They can be present in the same phase as the reaction educts (homogenous catalysts) or in a different phase (heterogeneous catalysts).
Methylpyridines are also used as organic solvents. Further, they are used in organic synthesis for producing derivatized products thereof. 3-picoline is a colourless, combustible liquid which is also used in the production of pharmaceuticals, dyes, rubber chemicals, resins and insecticides.
EP 0 770 687 B1 discloses the industrial synthesis of nicotinic acid amides, starting from 2-methyl-1,5-diaminopentane. This compound is converted to 3-methylpiperidine in the presence of a catalyst comprising an oxide of aluminium and/or silicon.
Subsequently, the 3-methylpiperidine is passed over a dehydrogenation catalyst and converted to 3-picoline. The 3-picoline is converted to 3-cyanopyridine with a further catalyst. Finally, the nicotinic acid amide is obtained in an enzymatic reaction.
In the art, various catalysts are known for dehydrogenating cyclic alkanes to arylic compounds. For instance, U.S. Pat. No. 4,401,819 discloses the use of palladium deposited on silica, alumina or carbon for the preparation of pyridine and substituted pyridines from piperidine and related compounds.
A method for preparing 3-methylpyridine from 3-methylpiperidine with a dehydrogenation catalyst is also disclosed in CN 1903842 A. In this process, the catalyst is based on palladium coated on a silicon dioxide carrier.
Specific catalysts for converting 3-methylpiperidine to 3-methylpyridine are also disclosed in WO 94/22824. The catalysts consist of palladium or platinum as the active component coated on a carrier comprising oxides of aluminium and/or silicon. In a specific embodiment, the dehydrogenation catalysts are obtained by impregnating silicon-aluminium oxide with a solution of a palladium-ammonia complex.
Thus, there is a continuing need for efficient processes for producing methylpyridines and for efficient catalysts, which are readily available. Specifically, there is a need for efficient catalysts which allow conversion of methylpiperidine to methylpyridine with a high yield. Further, there is a need for catalysts and processes, which keep the amount of undesired side-products low.
Another problem of palladium-based catalysts is that they can easily be inactivated by oxygen or other process chemicals (catalyst poisoning). Therefore, when such catalysts are used in an industrial process, their lifetime is limited. There is thus a need for catalysts, which are stable against inactivation and can be used in an industrial process for an extended time. Further, there is a need for processes for the production of methylpyridine, in which the conditions are adjusted such that catalysts can be used for a long time. The increase of catalyst lifetime is significant for reducing the costs of such a process, because palladium is an expensive precious metal. Further, the interruption times of the industrial continuous production process can be reduced when the catalyst is reactive over a long time period. Thus, costs can be kept low and product uniformity is preserved.